Materials are some of the drivers of the development of our society. This fact is well recognised in human history and a good example of this is that we have named key epochs, like the bronze age and the iron age, after the key material of the time. When it all began, humans used to collect ‘natural’ materials, and for a very long time, material selection criteria were based on fulfilling specific functions or simple criteria such as strength, hardness and weight in structural applications. The number and complexity of selection criteria rose dramatically when people discovered that the properties of natural materials can be altered significantly by changing their structure. Importantly, humans learned how natural oxides like iron oxide, which is rust, can be extracted into relatively pure metals, in this case iron. So, it became known that metals can not only be ground but also cast, cut and shaped into specific final products, and this was a huge step forward in material engineering. Humans also learned that strength can be increased and other properties altered dramatically by mixing one element with some others, and such mixtures became known as alloys. For instance, bronze is an alloy of copper and zinc, while steel is an alloy based on the mixture of iron and carbon. In essence, the advancement in making new structural and functional materials in this way was the beginning of an industrial revolution. It was a paradigm shift: instead of collecting natural materials and relying on their natural properties, we began designing and synthesising our own ‘artificial’ materials. The sophistication of modern engineering technologies allows the fabrication of a large variety of artificial materials that can fulfil a larger number of selection criteria simultaneously. The material selection criteria of today have expanded from the functions of materials alone to also include the characteristics of fabrication technologies, their cost, and the availability of natural resources. Today, we can fabricate materials that range from oxide ceramics and semiconductors to metals and polymers to composite and hybrid materials, and even to living biological tissues. For example, new strong lightweight material based structures allow aircraft to fly further, faster and even reach space, and new semiconductors now provide clean solar power. These are just a couple of examples of how society can benefit from materials innovation. However, the typical ‘side effect’ of fabricating artificial materials is their inability to decompose or degrade ‘naturally’ within a reasonable period of time. When we invest a lot of effort into materials taken from nature, very often an additional effort is necessary to make the materials safe to be returned. This ‘effect’ has been ignored by society for a very long time, but as modern scales of production grow along with population and consumption, the pollution arising from discarded complex materials has grown beyond the critical point. Therefore, this has come into the technological spotlight. But there are alternatives to discarding materials. Instead, the additional effort can be used to make the materials suitable for their original functions again. In other words, we can close the loops of ‘material cycles’. While this also requires effort, it has an added advantage that it also reduces overall material consumption and pollution. This brings us to a modern world where material technologists such as myself face new challenges. The basic one is to continue innovating to provide new materials with improved functionality. And the new one is to deliver materials that are also recyclable. This is a new paradigm for materials engineering, today, a material that delivers a function desired by society – but is also recyclable – is increasingly preferred. Not only ‘environmental friendliness’, but also ‘recyclability’, or a capacity to be a chain link in the emerging ‘circular economy’ is becoming crucial in material design and selection processes. This situation can be very well illustrated by the use of materials in the transportation industry, in particular in car bodies. Steel was an unrivalled structural material until the late 1970s when demands to reduce vehicle weight became stronger. This was to decrease fuel consumption and CO2 emissions and it spurred the replacement of some steel body components with light-weight alternatives. The result has been that many non load-bearing panels have been replaced by plastics. More critical components were replaced gradually by aluminium and then magnesium alloys as well as carbon-fibre composites. These special materials deliver significant weight reductions. The use of aluminium alloys allows weight saving in respective components for approximately 65%, while magnesium and carbon fibres save another 30%. But these light materials have their own drawbacks that go beyond the increased cost of a car production, that are related to the new material alloy and structure complexity required to achieve such performance. Both aluminium and magnesium as pure metals are very soft, and performance targets are achieved by making respective alloys. Without special additions, magnesium for example is notoriously difficult to fabricate – and it degrades too quickly during service. While aluminium alloys are extremely difficult – or simply too expensive – to recycle to their original grades. Therefore, they are ‘downcycled’ to lesser quality and value products. New composites such as carbon fibres have similar challenges. This brings us to the most interesting part of this example. The competition from light-weight materials stimulated accelerated development of better-performing steel grades. In this case, the weight saving is achieved by reducing component thicknesses. In addition, steel recyclability received an important stimulus as more valuable possibilities for ‘closing the loops of material cycle’ with steel were recognised. In turn, the revival of research in steel industries has further stimulated the development of light-weight materials. This has created more demands for the analytical capacities of sophisticated tools like electron microscopy, synchrotron radiation and neutron scattering. These tools combined allow us to design new materials satisfying the dual challenges of improved functionality plus circularity therefore opening fantastic opportunities for bringing the dreams of circular economy into life.